In general, an optical device includes a housing having a recess for housing an optical element, and a lid placed over an opening in the housing. The lid includes a transparent window. The housing and the lid are bonded together by low-melting glass or the like. Another optical device includes a package member for placing an optical element therein, and a metal cap combined with a window. The metal cap is placed on a base board, and the optical element is housed in the metal cap. The metal cap and the base board are jointed together by seam welding. With such structures, conventional optical devices each provide a sealed interior for housing an optical element.
For example, Patent Literature (PTL) 1 and PTL 2 disclose conventional techniques related to the present disclosure.